1. Field of the Invention
The present invention relates to a control method of call search (searching for a new perch channel), a mobile station and a mobile communications system including a mobile station and a plurality of base stations.
2. Description of the Related Art
A widespread mobile communications system such as a mobile telephone system provides services by dividing its entire service area into rather small radio zones. As shown in FIG. 1, for example, such a system comprises a plurality of base stations 111-1–111-5 covering divided radio zones, and mobile stations 112-1–112-3 carrying out communications with the base stations 111-1–111-5 by establishing radio channels between them.
Radio waves radiated from a base station at a certain transmission power propagates through space to a receiving site with attenuation. The attenuation the radio waves undergo increases with the distance between the transmitting site to the receiving site. Thus, perch channels from distant base stations are usually received at lower received power, whereas those from near base stations are received at higher received power. In practice, however, since the propagation loss changes depending on conditions such as topography and buildings as well as distance, the received powers of the perch channels from the base stations vary widely with the movement of the mobile stations. To receive signals transmitted from the base stations, it is important for each mobile station to always monitor the perch channels from the base stations and to select the best base station.
On the other hand, a technique called “intermittent reception” is applied to the mobile stations to increase the life of the batteries by saving power. Although the mobile stations in the standby mode must always monitor paging, the intermittent reception tries to minimize the power consumption by halting the receivers as long as possible when the reception is unnecessary. FIG. 2 is a schematic diagram illustrating a structure of a paging channel specified in “Specification of Air-Interface for 3G Mobile System Volume 3”. According to the specification, to increase the effect of the intermittent reception effect, a number of mobile stations are divided into a plurality of groups, and paging signals for individual groups are each mapped onto a single physical channel. FIG. 2 illustrates a paging signal for one of such groups. In this figure, symbols PI's (PI1 and PI2) designate a very short signal indicating the presence or absence of the paging, and MUI's (MUI1–MUI4) include paging information (ID number of the mobile stations). Each mobile station receives the PI portion first, and then receives the MUI portion only when a decision is made that the paging takes place as a result of receiving the PI portion. This makes it possible for the mobile station to reduce its consumption power to a minimum thanks to a small duty ratio of the reception because it is enough for the mobile station to carry out the two operations: receiving the paging for the group to which the mobile station belongs and receiving only the PI portion when no paging information is present. Although FIG. 2 illustrates the paging information from one of the base stations, which is decided and selected by the mobile station, the mobile station must carry out search for the perch channels from neighboring base stations while it is moving. Since the mobile station must receive many possible perch channels in the search operation for neighboring base stations, it is important for the mobile station to limit the frequency of the search operations to a minimum in order to increase the effect of the intermittent reception.
IMT-2000 (International Mobile Telecommunication-2000) standards specifying the third generation mobile communications system are now being planned in the 3GPP (Third Generation Partnership Project). The transmission schemes of the paging information are described in “3G TS 25.211 V3.2.0” which revises the “Specification of Air-Interface for 3G Mobile System Volume 3” a little to increase versatility. Specifically, the information indicating the presence and absence of the paging is transmitted over a PICH (Paging Indicator CHannel), and the paging information itself is transmitted over a SCCPCH (Secondary Common Control Physical CHannel). Although the physical configuration is thus changed, the procedure of the mobile station of receiving the PI portion first, followed by receiving the paging information only when a decision is made that the paging occurs as a result of the PI reception, and the effect of the intermittent reception achieved by the procedure are identical to those of the “Specification of Air-Interface for 3G Mobile System Volume 3”.
Thus, to select the best base station, it is necessary for each mobile station to always monitor the perch channels of the neighboring base stations by searching for and receiving them. On the other hand, the mobile station must reduce the duty ratio of its receiver to increase the life of the battery. Thus, there is an inevitable tradeoff between them. In other words, reducing the duty ratio of the receiver will bring about undesirable results such as prescribed service quality cannot be achieved because of the degradation in the accuracy of selecting the base station. In contrast with this, increasing the accuracy of selecting the base station by increasing the duty ratio of the receiver present an important problem of reducing the usefulness of the mobile station because of heavier consumption of its battery.
A document “Cell Selection Control in W-CDMA mobile station”, by Yunoki, Higashi and Tsutsumi, 1999 IEICE General Conference B-5-186 estimates the effect of the intermittent reception. It suggests that the mobile station must carry out periodic search for the neighboring base stations in order to implement proper selection of the base station taking account of the movement of the mobile station. However, since it is unavoidable for the periodic search for the neighboring base stations to determine such a search period that can achieve a certain level of the intermittent reception effect, a problem can arise in that the accuracy of selecting the base station is reduced in such conditions as the mobile station moves quickly or the propagation state changes sharply, because the search cannot follow such changes in those propagation environments. In contrast, in such conditions as the mobile station little moves or the propagation condition changes slowly, most of the perch channel search becomes needless, which presents a problem of wasting the battery of the mobile station.